The invention relates to an apparatus for generating vibrations in a pipe string, such as a drill string carrying a rotary drill bit, in a borehole penetrating subsurface earth formations.
When drilling highly deviated holes and/or horizontal holes with or without drill string rotation, the gravity force acting on a length of drill pipe as it lies on lowside of hole, when resolved in the direction of the hole, is insufficient to overcome friction in order to advance the drill string as the bit drills off bit weight. Thus there is a need for an apparatus which is able to move a drill pipe string through a borehole in case friction between the borehole wall and the string is high. It may also be needed to compact a gravel packing or cement lining by vibration, or to fish a stuck drill string or other tubulars, such as production liners or casing strings, gravel pack screens, etc., from a borehole.